Certain effluent gas streams contain many toxic pollutants, including the heavy metals mercury and cadmium. Coal-burning power plants, incinerators, oil-burning boilers and power plants, refuse-derived fuel power plants, and gasification systems (especially integrated gasification combined cycle power generation systems) are sources of effluent streams with mercury and other heavy metals. The combustion of low-rank coals, such as Powder River Basin sub-bituminous coal and lignites, has been shown to form flue gases where the mercury is primarily in the elemental form. Indeed, much of the mercury contained in power plant flue gas is in the elemental form. Elemental mercury is a semi-noble metal, insoluble in water, and is not efficiently captured by carbon. As such, elemental mercury is difficult to capture while oxidized mercury is more amenable to capture.
Mercury compounds, such as mercury (II) chloride, HgCl2, are highly soluble in water and more readily removed by carbon sorbents. Therefore, agents that can oxidize elemental mercury to mercury (II) chloride (or another oxidized mercury compound) would be of considerable value in effluent clean-up. Such agents would enable mercury to be captured by existing air pollution control devices (APCDs) present at coal-burning power plants. Typical APCDs include wet and dry scrubbers for acid gas removal (SOx and NOx), as well as electrostatic precipitators (ESPs) and baghouse filters for particulate removal.
Ideally, the mercury oxidation catalysts would be located upstream of the appropriate APCD. Mercury (II) chloride is readily removed by the scrubbing solutions employed for acid gas removal and/or by adsorption on unburned carbon in fly ash captured by ESPs or baghouse filters. Mercury (II) chloride is also sequestered by activated carbon sorbents injected upstream of an ESP or baghouse.
Many technologies are being developed for the control of mercury emissions from flue gases. These methods employ sorbents, catalysts, scrubbing liquors, flue gas or coal additives, combustion modifications, barrier discharges, and ultraviolet (UV) radiation. Efforts have been made to develop catalysts for selectively oxidizing elemental mercury in effluent streams. However, these selective catalytic reduction (SCR) catalysts, whose primary role in flue gas is the reduction of nitrogen oxides, typically achieve only 50 percent oxidation levels for mercury oxidation. SCR catalysts have not yet been optimized for mercury oxidation and removal.
Due to its relative simplicity and proven successful application for the incinerator market, most of the research has focused upon the use of activated carbon sorbents for the adsorption of mercury.
Gold (Au) and palladium (Pd) catalysts are used as mercury oxidation catalysts. For example, U.S. Pat. No. 6,136,281 awarded to Meischen, et al. on Oct. 24, 2000, discloses a gold catalyst for the oxidation of elemental Hg in effluent gas streams. However, both Au and Pd catalysts are not inert and therefore degrade in flue gases.
Thief carbons and their adsorption capabilities are described in U.S. Pat. No. 6,521,021 awarded to Pennline, et al., on Feb. 18, 2003, which is incorporated in its entirety by reference herein.
None of the aforementioned patents discloses catalysts with both high mercury oxidation levels and high adsorption levels for HCl, Cl2, and other halogens or halogen-containing compounds. In addition, none of the aforementioned patents disclose a catalyst which is self-activating and disposable.